SE power tube switch

[jeff]

 I've read a while back that if you have a 4 tube push pull amp instead of pulling out 2 tubes you can turn them off by disconnecting the cathode to ground.

 Is it possible to wire a SE amp where you wire a EL84 and a 6V6's plates, grids and screens in parallel but only ground one of the tube's cathode resistor at a time to switch which one's on?

 If so how would the "off" tube effect the "on" tube? In other words is a 6V6 with it's plate, screen, and grid connected to a EL84 operating electronically the same as when it is not connected in parallel to another tube?

[PRR]

Yes.

6V6 & EL84 need different cathode resistors, of course.

If you are not in a hurry, switching the heater saves power.

A cold tube is just bits of metal in an empty bottle.

A hot cathode tube with NO connection to cathode is nearly the same.

[John]

That is a cool idea! What advantages would there be between the 2? More volume and/or different tone between them? But, don't the 2 different tubes also require/use different B+ voltages?

[jeff]

Yes.

6V6 & EL84 need different cathode resistors, of course.

If you are not in a hurry, switching the heater saves power.

A cold tube is just bits of metal in an empty bottle.

A hot cathode tube with NO connection to cathode is nearly the same.

 I was under the impression that a cold tube with B+ voltage was bad for the tube, no?
Isn't that the point of a standby switch, so the tube doesn't get the B+ 'til it's hot?

 Also wouldn't you be pulling more current as one tube cools and the other heats up, Both will be conducting at the same time for a short time?

[jeff]

That is a cool idea! What advantages would there be between the 2? More volume and/or different tone between them? But, don't the 2 different tubes also require/use different B+ voltages?

Don't know but I thought this would be a good idea for a taste test (A/B)

[billcreller]

Don't know if this sounds correct, but a double pole switch could shut down both the cathode and heater, if a switch was used for each tube.

[HotBluePlates]

... What advantages would there be between the 2? More volume and/or different tone between them? But, don't the 2 different tubes also require/use different B+ voltages?

Carefully look over the data sheets for each of these tubes, comparing the conditions for same/similar screen and plate voltage.

You will find similar/same OT loading, because they are both 12w tubes (okay, later 6V6GTA's became 14w, but often the sheets were not updated to show that).

What you will see is that the EL84 has very much more transconductance. That means with less voltage swing on G1, you get bigger plate current swings. It takes less input to get the same output.

So the EL84 should make full power and break into distortion faster than the 6V6. Not at lower volume because they are rated for the same power, but at a lower setting of your amp's volume knob.

The other effect of higher Gm is that the EL84 requires less voltage to properly bias, so it needs a smaller cathode resistor to idle at a happy condition. Which is why PRR says 2 sockets, 2 seperate cathode resistors.

[John]

Ah, now I understand. Thanks for the explanation!